When faced with a similar situation, here's how I typically address the issue:
1. I create an external-source DataWindow with only the example column(s) I need, and set the column properties in the Edit tab. In your case, I'd define a single column and I would set the Edit Style for that column to Drop-down List Box and define the same set of display and data values that I'm wanting to set dynamically via code. Save the DataWindow.
2. I Edit Source for the newly-created DataWindow and locate the syntax that I'm interested in. In this particular instance, the code table that contains the display & data values used in the DDLB edit style is included in the description of the column definition (not the column DWObject) near the top of the source syntax. Look for: values=".....".
3. In the Values property string, make special note of the use of a tab character between each display and data value, and the forward slash (/) that delimits each display/data value pair - including the end of the last pair. Of course, in a Modify string, you would use ~t instead of a 0x09 character. Syntactically, everything has to be just right for the Modify to work.
Best regards, John